IMR-MS device

ABSTRACT

The present invention relates to an apparatus and a method for IMR-MS and/or PTR-MS, comprising a sample gas inlet ( 202, 206 ), a first ion source ( 209 ), a reaction chamber ( 203 ), a mass analyzer ( 204 ), wherein the reaction chamber ( 203 ) and the mass analyzer ( 204 ) are arranged along a central axis (A), characterized by a second ion source ( 209 ), wherein the sample gas inlet ( 202, 206 ) is arranged to introduce gas essentially along the central axis (A) and is connected to the reaction chamber ( 203 ); wherein the first ion source ( 209 ) and the second ion source ( 209 ) are arranged so as to emit reagent ions essentially perpendicularly to the central axis (A); said apparatus further comprising at least one electrode ( 302, 303, 304, 305 ), such that the reagent ions emitted from the first or second ion source ( 209 ) can be deflected into the reaction chamber ( 203 ) essentially in the downstream direction of the central axis (A).

The present invention relates to an apparatus for Ion-Molecule-ReactionMass Spectrometry and/or Proton-Transfer-Reaction Mass Spectrometry,comprising a sample gas inlet, an first ion source, a reaction chamber,a mass analyzer, wherein the reaction chamber and the mass analyzer arearranged along a central axis. The invention further relates to a methodto operate an apparatus for Ion-Molecule-Reaction Mass Spectrometryand/or Proton-Transfer-Reaction Mass Spectrometry according to theinvention.

BACKGROUND OF THE INVENTION

Ion-Molecule-Reaction Mass Spectrometry (IMR-MS) orProton-Transfer-Reaction Mass Spectrometry (PTR-MS; both terms are usedsynonymously throughout this application) is a well-established methodfor chemical ionization, detection and quantification of (trace)compounds. Details about the technology can e.g. be found in A. M.Ellis, C. A. Mayhew (Proton Transfer Reaction Mass SpectrometryPrinciples and Applications, John Wiley & Sons Ltd., UK, 2014).Advantages of this technique are high sensitivity, high selectivity,on-line quantification, direct sample injection and short responsetimes. Although most common PTR-MS instruments employ proton transferfrom H₃O⁺ to the analytes, the technology is by no means limited to thisform of ionization. Several instruments have been introduced, whichenable the use of NO⁺, O₂ ⁺, Kr⁺ and any other type of positively ornegatively charged reagent ions for chemical ionization. In addition toa series of common devices for controlling the various voltages,currents, temperatures, the vacuum, etc., a typical PTR-MS instrumentcomprises the following main components:

Reagent Ion Source:

In the reagent ion source the reagent ions are formed. Many PTR-MSinstruments employ a hollow cathode ion source fed by suitable sourcegases (e.g. H₂O vapor, O₂, N₂, noble gases, etc.), but various otherdesigns have been introduced (e.g. point discharge, plane electrodedischarge, microwave discharge, radioactive, etc.). Favorable ionsources produce reagent ions of high purity, either because of theirsophisticated design or because of the use of mass filters.

Reaction Chamber/Drift Tube:

In the PTR reaction chamber chemical ionization of the analytes takesplace via interactions with the reagent ions. The reaction chamber isoften also referred to as drift tube or reaction region. While a certainflow of gas containing the analytes is continuously injected, anelectric field draws ions along the drift tube. Commonly, air containingtraces of impurities (e.g. traces of volatile organic compounds) isanalyzed by PTR-MS, but many other matrices containing compounds ofinterest (e.g. remaining impurities in purified gases, gas standards,etc.) have been successfully investigated with various reagent ions. Insome embodiments the matrix containing the analytes (e.g. air withtraces of volatile organic compounds) is diluted with a buffer gas priorto injection into the drift tube (e.g. for simple dilution purposes, forthe use of particular reagent ions, etc.).

Some of the common reactions between the reagent ion and the analytetaking place in the reaction chamber are:

-   -   Proton transfer reactions, either non-dissociative or        dissociative, with A.H⁺ being the reagent ion (in most cases        H₂O.H⁺) and BC being the analyte:        A.H ⁺ +BC→A+BC.H ⁺        A.H ⁺ +BC→A+B+C.H ⁺    -   Charge transfer reactions, either non-dissociative or        dissociative, with A⁺ being the reagent ion (e.g. O₂ ⁺, NO⁺,        Kr⁺, etc.) and BC being the analyte:        A ⁺ +BC→A+BC ⁺        A ⁺ +BC→A+B+C ⁺    -   Clustering reactions, with A⁺ being the reagent ion (e.g. H₃O⁺,        NO⁺, etc.) and BC being the analyte:        A ⁺ +BC→BC.A ⁺

In addition other types of reactions can occur (e.g. ligand switching,H⁺ extraction in case of negatively charged reagent ions, etc.).

Most common drift tubes consist of a series of ring electrodeselectrically connected via resistors with equal resistance, so that a DCvoltage U can be applied across a drift tube of the length d, resultingin the electric field strength E=U/d (in V/cm).

Mass Analyzer and Detector:

Between the reaction chamber and the mass analyzer there is a transitionregion to account for the pressure difference between these two regions,as mass analyzers typically operate in high or ultra high vacuumregimes. Various types of mass analyzers have been employed in PTR-MSinstruments. The most prominent example for a low resolution massanalyzer is the quadrupole mass filter, whereas for high mass resolutionmeasurements Time-Of-Flight (TOF) analyzers are commonly used in PTR-MS.However, the use of other types of mass analyzers, such as e.g. ion trapanalyzers, has also been reported and even MSn (multiple-stage massspectrometry) could be realized. The mass analyzer separates the ionsinjected from the drift tube according to their m/z and quantifies theion yields of the separated m/z with a suitable detector (e.g. secondaryelectron multiplier, microchannel plate, etc.).

STATE OF THE ART

One of the first PTR-MS instruments, which was introduced already in1995 (A. Hansel, A. Jordan, R. Holzinger, P. Prazeller, W. Vogel, W.Lindinger, Proton transfer reaction mass spectrometry: on-line trace gasanalysis at the ppb level. International Journal of Mass Spectrometryand Ion Processes 149/150 (1995) 609-619) employed a hollow cathodereagent ion source in-line (on the same central axis) with the adjacentdrift tube. Perpendicular to this axis the sample inlet was mounted atthe beginning of the drift tube. As this early concept is stillstate-of-the-art in the vast majority of the many hundreds of PTR-MSinstruments in use nowadays, it is discussed in some more detail insection “Detailed description of the invention”.

Two rare exceptions to the common design have been published byBreitenlechner (An Instrument for Studying the Lifecycle of ReactiveOrganic Carbon in the Atmosphere. Analytical Chemistry 89 (2017)5824-5831) and Krechmer (Evaluation of a new vocus reagent-ion sourceand focusing ion-molecule reactor for use in proton-transfer-reactionmass spectrometry. ChemRxiv (2018) preprint). Breitenlechner et al.developed a novel type of PTR-MS instrument specifically designed foratmospheric chemistry with extremely high sample gas flow rates. Theiraim was to have the sample inlet as much in the direction of thereaction chamber central axis as possible. However, this inevitably ledto a conflict with the position of their corona discharge reagent ionsource, which should also be in line with the reaction chamber centralaxis. The solution they came up with is an embodiment where both, theion source and the sample inlet, point in the direction of the reactionchamber central axis at a slight off-axis angle. Krechmer et al. came upwith a different design where the inlet line points exactly in directionof the reaction chamber central axis and the ion source consists of twoconical surfaces (with a plasma burning in between) surrounding theinlet line, thus pointing in direction of the reaction chamber centralaxis in a slight off-axis angle.

Another example dealing with the conflict between having the ion sourceand the sample inlet pointing essentially in the direction of thecentral axis of the reaction chamber is given in U.S. Pat. No. 7,095,019B1. There, a different type of instrument, which is not a PTR-MSinstrument, is described and the conflict is solved by placing the ionsources at an angle of about 45° to the central axis. This choice leadsto very limited space in the sample inlet region.

Furthermore, WO 2018/050962 A1 discloses a general MultimethodIonization Device to utilize chemical ionization and a system utilizingsuch a device provided with a reaction chamber for ion formation ofreagent species, which is again not a PTR-MS instrument. The aim of theionization instrument in WO 2018/050962 A1 is to detect substances withvery high sensitivity, which is achieved by locating several ionizationsources in and around the reaction region. The instrument in WO2018/050962 A1 can answer the question if a certain substance is presentin the sample, but cannot quantify the compound and is thus not a PTR-MSinstrument.

Two major problems occur with PTR-MS reagent ion source designs so far:

a) Although switching reagent ions, e.g. from H₃O⁺ to O₂ ⁺, has beenreported to be relatively rapid, it still takes a considerable amount oftime. Based on literature reports and the inventors experience, afterabout 3 to 4 seconds the main switching process has been completed (massflow controllers have switched the source gas, the gas in the ion sourcehas partly been replaced and the voltages and pressures have beenchanged). However, in order to get reagent ions of high purity up totens of seconds are necessary, e.g. for getting rid of remaininghumidity in the ion source. Directly compared to technologies like e.g.Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS), where a quadrupolemass filter is employed to select the reagent ions and switching can beperformed within split-seconds, this can be considered as a majordrawback of PTR-MS. Installing a mass filter similar to SIFT-MS is notan option for PTR-MS as mass filters never can achieve 100% transmissionefficiency and therefore one of the major advantages of PTR-MS would belost: extremely high sensitivity. No satisfying solution has beenproposed to this problem so far.

b) According to the prior art for proper performance it is necessarythat the ion source as well as the sample inlet are aligned with thereaction chamber's central axis. Since two devices cannot be at the sameplace at the same time, such an arrangement is difficult. The reason forthis alignment is that

-   -   i) an extremely high reagent ion current is necessary to achieve        extraordinary sensitivity of the PTR-MS instrument (ion source        should inject reagent ions into the central axis of the reaction        region) and    -   ii) contact of the sample gas to inlet line walls should be        avoided because of possible condensation and conversion effects        (i.e. a straight inlet without corners, T-pieces, valves, mass        flow controllers, etc. is beneficial).

SHORT DESCRIPTION OF THE INVENTION

The object of the present invention is thus to provide an IMR/PTR-MSdesign which solves the problems mentioned in a) and which meets therequirements mentioned in i) and ii), while introducing no drawbacks.

This object is solved by an apparatus for Ion-Molecule-Reaction MassSpectrometry and/or Proton-Transfer-Reaction Mass Spectrometry,comprising

-   -   a sample gas inlet,    -   an ion source section,    -   a first ion source,    -   a reaction chamber,    -   a mass analyzer,

wherein the reaction chamber and the mass analyzer are arranged along acentral axis, characterized by a second ion source,

-   -   wherein the sample gas inlet is arranged to introduce gas        essentially along the central axis into the ion source section        and is connected to the reaction chamber;    -   wherein the first ion source and the second ion source are        arranged essentially in a plane in front of the reaction chamber        so as to emit reagent ions essentially perpendicularly to the        central axis into the ion source section;    -   said apparatus further comprising at least one electrode, such        that the reagent ions emitted from the first or second ion        source into the ion source section can be deflected into the        reaction chamber essentially in the downstream direction of the        central axis.

The notations “central axis” and “axis of the reaction chamber” are usedessentially synonymously. The notation “in the downstream direction ofthe central axis” means “in the direction of the central axis and indownstream direction”.

This object is further solved by a method to operate an apparatus forIon-Molecule-Reaction Mass Spectrometry and/or Proton-Transfer-ReactionMass Spectrometry according to one described, characterized by thefollowing steps:

-   -   introducing the sample gas into the ion source section via the        sample gas inlet in the direction of the central axis of the        reaction chamber;    -   continuously generating at least two different reagent ions in        the at least two ion sources, wherein in one respective ion        source one specific type of reagent ions is produced;    -   applying a voltage to the at least one electrode at the exit of        each ion source, wherein a certain value of voltage leads to        injection of the respective reagent ions into the ion source        section and another certain value of the voltage leads to        rejection back into the respective ion source;    -   applying a voltage to the at least one electrode positioned        upstream in the vicinity of the sample gas inlet, wherein this        electrode causes a repulsive force onto the reagent ions and        applying a voltage to the at least one electrode positioned        downstream in the vicinity of the reaction chamber, wherein this        electrode causes an attracting force onto the reagent ions, such        that the reagent ions are injected into the reaction chamber        essentially in the downstream direction of the central axis of        the reaction chamber;    -   introducing the reagent ions and/or the sample gas into the        reaction chamber, wherein subsequent to the reaction chamber the        analyte or the sample gas are analyzed with the mass analyzer.

With respect to the apparatus there are several preferred embodiments.

In a preferred embodiment the apparatus is characterized by at least onefurther ion source arranged so as to emit reagent ions essentiallyperpendicularly to the central axis.

Regarding the orientation of the at least two ion sources, the axes(e.g. central axis or longitudinal axes) of the at least two, preferablyat least three, ion sources are essentially perpendicular to the centralaxis of the reaction chamber. In a preferred embodiment the ion sourcesare arranged essentially in a plane which is essentially perpendicularto the central axis of the reaction chamber.

The apparatus may be further characterized in that said electrodes areconnected to a switching device so that emitted reagent ions from oneion source are deflected into the central axis while reagent ions fromany other ion source are rejected back into the respective ion source.

Preferably the ion sources are positioned in the area of the ion sourcesection, wherein the reaction chamber is downstream and preferablyadjacent to the ion source section.

In a preferred embodiment the apparatus comprises three ion sources,wherein a first ion source is capable to produce H₃O⁺ out of H₂O vapor,a second ion source is capable to produce O₂ ⁺ out of O₂ and a third ionsource is capable to produce NO⁺ out of N₂ and O₂. The angle betweenadjacent ion sources can measure essentially 120 angular degrees.

Preferably the ion sources are hollow cathode ion sources.

In a preferred embodiment it is provided that the sample gas inletexhibits a first part of the sample gas inlet and a second part of thesample gas inlet, wherein the second part of the sample gas inlet isadjacent to the first part of the sample gas inlet. Preferably thesample gas inlet is in the direction of the central axis of the reactionchamber, wherein the sample gas inlet is essentially parallel to thecentral axis, preferably in immediate vicinity to the central axis.

Particularly preferred the second part of the sample gas inlet isdownstream the first part of the sample gas inlet, wherein the diameterof the second part of the sample gas inlet is preferably less than thefirst part of the sample gas inlet, wherein the first part and thesecond part of the sample gas inlet are fluidically connected.

Preferably the second part of the sample gas inlet is fluidicallyconnected with the ion source section and the ion source section isfluidically connected with the reaction chamber.

In an embodiment the apparatus comprises a sample inlet bypass line,wherein the sample inlet bypass line is arranged essentiallyperpendicular to the sample gas inlet. In a two-part sample gas inletthe sample inlet bypass line is preferably fluidically connected withthe first part of the sample gas inlet.

Preferably a gas line is arranged essentially perpendicular to thesample gas inlet. In a two-part sample gas inlet the gas line ispreferably fluidically connected with the second part of the sample gasinlet.

A particularly preferred embodiment provides that the ion source sectionconsists of at least two electrodes, preferably of at least threeelectrodes, wherein at least one electrode is positioned opposite toeach ion source and/or at least one electrode is positioned in theimmediate vicinity of the second part of the sample gas inlet and/or atleast one electrode is positioned in the immediate vicinity of thereaction chamber.

The electrodes constitute a chamber-like entity referred to as the ionsource section, comprising ion sources outside of the chamber-likeentity, wherein the ion sources are fluidically connected with theinside of the chamber-like entity. Preferably the axes of the ionsources point essentially to the center of the ion source section.

The at least two ion sources can be positioned between the electrodewhich is positioned in the immediate vicinity of the second part of thesample gas inlet and the electrode which is positioned in the immediatevicinity of the reaction chamber.

Preferred the at least two ion sources comprise at least one electrode,with which the injection of reagent ions into the ion source sectionand/or the rejection back into the respective ion source is feasible.

The sample gas inlet is preferably arranged along the central axis andleads to the reaction chamber.

In one embodiment the at least one electrode is positioned opposite toeach ion source and/or at least one electrode is positioned upstream inthe vicinity of the sample gas inlet and one electrode is positioneddownstream in the vicinity of the reaction chamber.

Preferably each ion source comprises at least one electrode at the exit.

With respect to the method it can be provided that a voltage is appliedto the at least one electrode opposite to the at least two ion sources,wherein said electrode causes a repulsive force onto the reagent ions.

Particularly preferred a controlling device controls the voltagesapplied to the electrodes, wherein the controlling device controls whichkind of the reagent ions generated in the respective ion source will beinjected into the ion source section and/or which kind of reagent ionsgenerated in the respective ion source will be rejected back into therespective ion source.

In a special variant, the controlling device adapts the parameters ofthe apparatus, such that the apparatus on demand acts as an AtmosphericPressure interface Mass Spectrometer, wherein the parameters comprise atleast the voltages applied to the electrodes, wherein the controllingdevice controls that the ions generated in the ion sources are rejectedback into the respective ion sources or the ion sources are switched offby the controlling device such that no reagent ions are generated.

Further embodiments and advantages are explained by reference to thefigures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic view of the prior art of IMR/PTR-MSinstruments.

FIG. 2 shows a schematic view of an exemplary IMR/PTR-MS instrumentaccording to the present invention.

FIG. 3 shows a schematic view of an exemplary reagent ion sourcearrangement with three ion sources.

FIG. 4 shows a schematic view of the ion source section according to thepresent invention.

FIG. 5 shows a schematic illustration of an embodiment where one reagention source is set to inject reagent ions into the ion source section andone reagent ion source is set to block ions from entering the ion sourcesection.

FIG. 6 shows a schematic view of a prototype built according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic overview of the prior art, with a reagent ionsource comprising a first part 101 and a second part 102, a drift tube103, a mass analyzer 104 and a sample inlet 105. Water vapor originatingfrom a reservoir filled with purified water enters the first part 101 ofthe ion source. In the first part 101 a hollow cathode dischargeconverts H₂O molecules into a series of product ions: H₂O⁺, H⁺, H₂ ⁺,OH⁺ and O⁺. As eventually highly pure H₃O⁺ reagent ions are needed inthe second part 102 of the ion source also called “source drift” regionthe product ions from the first part 101 and the water vapor, which areboth transferred into the second part 102 via gas flow and/or electricfields, undergo various ion-molecule reactions which lead to H₃O⁺ puritylevels of 99% and above. The underlying ion-chemistry has been discussedin detail in literature. Subsequently, the H₃O⁺ reagent ions areinjected into drift tube 103, where they can interact with the gascontaining the analytes (e.g. air with trace compounds) introduced intothe drift tube via sample inlet 105.

It has been shown that this ion source design can also be used for theproduction of very pure reagent ions apart from H₃O⁺, e.g. NO⁺, O₂ ⁺,Kr⁺, Xe⁺, NH₄ ⁺, OH⁻, etc. by switching the source gas and adjusting thecurrents and voltages applied to and the pressure in the ion source.

The invention relates to a front end of an IMR/PTR-MS instrument,denoted 201 in FIG. 2. Sample gas is drawn into the instrument via asample gas inlet 202, 206 (also termed as sample inlet), which isessentially in direction of the central axis A of the reaction chamber203 to the mass analyzer 204. A sample inlet bypass line 205 allows forregulating the sample inlet flow while keeping the flow into thereaction chamber 203 constant. That is, a vacuum pump (membrane pump,scroll pump, multi-stage turbomolecular pump, etc.) is connected to thesample inlet bypass line 205.

In a preferred embodiment a mass flow controller, valve or similardevice which allows for regulating the air flow is installed between thesample inlet bypass line 205 and the vacuum pump. If this flowregulating device is completely closed so that no suction is created viasample inlet bypass 205 only a minimum amount of gas is sampled (thesample gas flow which enters the reaction chamber 203). If the flowregulating device is opened, virtually any sample inlet flow higher thanthe minimum can be set. Typically IMR/PTR-MS reaction chambers areoperated between 0.1 and 100 hPa, preferably between 1 and 10 hPa. Thisvacuum is usually maintained by one or more vacuum pumps connected toone or more pumping ports of the reaction chamber 203 and determines theminimum gas flow needed for operating the instrument, i.e. the pressurein 203 results from the gas entering and being pumped away. In line ofcentral axis A is also the second part of the sample gas inlet 206. Thispart, which is adjacent to the connection point of the sample inletbypass 205, preferably has a smaller inner diameter than the first partof the sample gas inlet 202, so that the gas flow through 206 isrestricted.

Gas line 207 is connected to the second part of the sample gas inlet 206and to a vacuum pump, which can be the same vacuum pump that isconnected to 205 or an additional pump of the same or different type.Preferably a pressure controller, a valve or any gas flow regulatingdevice is interconnected between gas line 207 and the vacuum pump. Viathe gas flow/suction through gas line 207 the pressure in the reactionchamber 203 (which itself is evacuated by a vacuum pump) can beregulated.

The sample gas flows into the ion source section 208. This section canbe part of the reaction chamber 203 or a separated section. At least tworeagent ion sources 209 are mounted in this section essentiallyperpendicular to axis A, i.e. in plane B. The ion sources 209 can be anyIMR/PTR-MS reagent ion sources (e.g. point discharge, plane electrodedischarge, microwave discharge, radioactive, etc.). In a preferredembodiment the ion sources 209 are hollow cathode ion sources.

FIG. 3 shows a schematic view of an exemplary embodiment in direction ofcentral axis A. Here, three reagent ion sources 209 are mounted at 0°,120° and 240° (angular degrees) in plane B, which is essentiallyperpendicular to axis A. In this exemplary embodiment the ion sourcesection 208 has a circular cross section. Any other cross section isalso possible, e.g. triangular, rectangular, polygonal, elliptic, anycombination of curved and/or straight forms. Any positions of the ionsources 209 are possible, e.g. opposite of each other or at any angle toeach other. The number of ion sources 209 has to be at least two.

FIG. 4 shows a schematic view of an exemplary embodiment of the ionsource section 208. In this figure the sample gas is introduced from theleft via orifice 301 (connected to sample gas inlet 206, not shownhere). A DC voltage can be applied to electrode 302. At position 303 isat least one electrode with an orifice (ion lens) where a DC voltage canbe applied. By applying an appropriate voltage to electrode 303 thereagent ions generated in the reagent ion source 209 can be injectedinto the ion source section 208 or rejected, so that they do not enterthe ion source section 208. Electrode 304 can either be the firstelectrode of the IMR/PTR-MS drift tube or an ion lens for injectingreagent ions into the reaction chamber. 305 is an electrode opposite ofthe reagent ion source, which can e.g. be a metal plate or theelectrically conducting inner housing of the ion source section 208. Insome embodiments no electrode 305 is present. In a preferred embodimentthe function of electrode 305 is substituted or supplemented byelectrode 303 of the at least one additional ion source 209 other thanthe ion source 209 currently injecting reagent ions. By applyingappropriate electric potentials/voltages to 302, 303, 304 and 305reagent ions are guided into the IMR/PTR-MS reaction chamber asschematically indicated by arrow 306.

If according to the present invention at least two reagent ion sources209 are installed in the ion source section 208, reagent ions can beselected by simply changing the voltages applied to the electrodes.Preferably the voltages that are changed are the voltages of the ionsource exit lenses 303.

In FIG. 5 two reagent ion sources are schematically shown, where thereagent ions of one ion source are rejected (do not enter the ion sourcesection and eventually the IMR/PTR-MS reaction chamber) and the reagentions produced by another ion source are injected into the ion sourcesection and eventually the IMR/PTR-MS reaction chamber. That is, if e.g.one reagent ion source continuously produces one type of reagent ionsand a second reagent ion source continuously produces another type ofreagent ions, the reagent ions used for chemical ionization in theIMR/PTR-MS reaction chamber can be very rapidly switched by simplychanging electrode voltages. In stark contrast to existing designs, notime consuming source gas switching or pressure adjustments have to beperformed. It has been shown that the reagent ion yields entering thereaction chamber are of comparable intensity to reagent ion yields of acommon single ion source in-line with the reaction chamber's centralaxis, i.e. intensity losses due to the perpendicular position arenegligible.

In a preferred embodiment at least three reagent ion sources areinstalled, which continuously produce at least H₃O⁺, NO⁺ and O₂ ⁺,respectively.

In another embodiment at least four reagent ion sources are installed,which continuously produce at least H₃O⁺, NH₄+, NO⁺ and O₂ ⁺,respectively.

Obviously, if for a prolonged measurement it is foreseeable thatswitching of the reagent ions will not be required, i.e. only one typeof reagent ions is necessary, all reagent ion sources except for the oneproducing the required reagent ions can be turned off in order to savesource gas and prevent wear effects. However, in this case no rapidswitching is possible.

In another embodiment all reagent ion sources are turned off or set sothat they do not inject reagent ions into the reaction region. In thismode of operation the novel front end acts as an APi(Atmospheric-Pressure interface) device and atmospheric ions can beanalyzed with the IMR/PTR-MS instrument. In this case no chemicalionization reactions between reagent ions and the sample gas take placewithin the reaction region, but the reaction chamber only transports theatmospheric ions to the mass analyzer. Again, only electric potentialsand voltages have to be changed to enable this mode of operation so thatthe atmospheric ions are guided into the reaction region andsubsequently into the mass analyzer.

Exemplary Embodiment

A schematic view of a prototype embodiment, which only acts as anexample and should by no means limit the invention to this embodiment,is displayed in FIG. 6.

In an inlet block 401 an opening 402 is drilled in direction of thecentral axis A. This opening 402 is equipped with a thread to screw in a1/16 inch sample inlet line (e.g. made of PEEK (PolyEther Ether Ketone),passivated stainless steel, PTFE, etc.) with 1 mm inner diameter. Anopening 403 is drilled perpendicular to opening 402 and connected to amembrane vacuum pump via a mass flow controller. By adjusting the massflow controller the amount of gas containing the analytes sampled by theinstrument can be adjusted from the minimum which is needed foroperation of the IMR/PTR-MS instrument (between 10 and 100 cm³/min atstandard conditions for the prototype instrument) to the maximum pumpingpower of the membrane vacuum pump. Adjacent to the drilling 402 there isa drilling 404 with a smaller diameter in direction of the central axisA. Perpendicular to drilling 404 there is a drilling 405 which isconnected to the same vacuum pump as drilling 403, but with aninterconnected pressure controller instead of a mass flow controller. Byadjusting this pressure controller the (resulting) pressure in theIMR/PTR-MS reaction chamber can be adjusted. The second part of theinlet line 404 ends in an electrode/ion lens 406 at the beginning of theion source section 407.

Three hollow cathode reagent ion sources 408, 409 and 410 (the lattertwo are only indicated in the schematic view) are mounted perpendicularto central axis A in plane B at 120° offset angle, respectively (compareFIG. 3). All three reagent ion sources are of the same design: A firstionization chamber 411 and a second ionization chamber 412. Bothionization chambers are essentially made of a conductive material.Preferably the conductive material is stainless steel, such as stainlesssteel type EN 1.4301, 1.4405 or 1.4407. One or more source gases areintroduced via mass flow controllers into the first chamber 411, where ahollow cathode discharge ionizes the source gas. Typical source gasesare H₂O vapor for H₃O⁺, O₂ for O₂ ⁺, a mixture of N₂ and O₂ for NO⁺ anda mixture of N₂ and H₂O vapor for NH₄ ⁺ production. Ions and neutralssubsequently enter a second ionization chamber 412 where they react viaion-molecule reactions and highly pure reagent ions of one particulartype are formed. Ionization chamber 412 is connected to a vacuum pumpvia an electronically controlled (proportional) valve so that thepressure can be regulated. Electric fields can be applied to ionizationchambers 411 and 412 in order to control the hollow cathode discharge,transport the ions and control the ion-molecule reactions. At least oneion lens at the exit of chamber 412 into the ion source section 407enables blocking (rejecting) or transmitting the ions.

Each of the three reagent ion sources 408, 409 and 410 continuouslyproduces one particular type of reagent ions. However, only the voltageapplied to the exit ion lens of one ion source is set so that thesereagent ions can enter the ion source section 407. The remaining tworeagent ion types are hindered from entering the ion source section 407by the voltage applied to the exit ion lenses. After entering the ionsource section 407 the reagent ions are drawn in direction of centralaxis A by electric fields. These fields are e.g. created by electrode406 and the ring electrodes of the reaction chamber 413.

Additionally, the electrodes at the exits of chambers 412 can createelectric fields which penetrate to some extent into the ion sourcesection 407. That is, at the exit electrodes of those reagent ionsources which are set so that they do not inject reagent ions into theion source section 407 a repulsive electric potential is applied to.This repulsive potential hinders reagent ions from exiting therespective ion sources.

On the other hand, it pushes reagent ions originating from anotherreagent ion source towards central axis A. In other words the reagentions which are injected into the ion source section 407 from one“active” reagent ion source 408 are repelled by electrode 406 and theexit electrodes of the other two “inactive” reagent ion sources anddrawn into the reaction chamber 413. For example, if the reagent ionsource at 0° (angular degrees) produces H₃O⁺ reagent ions and injectsthese reagent ions via acceleration due to at least one electrode at theexit of chamber 412 into the ion source section 407, the exit electrodesof the remaining two reagent ion sources at 120° and 240° are set sothat they do not inject reagent ions into the ion source section 407.The repulsive electric field created by these two electrodes helps tokeep the H₃O⁺ reagent ions originating from the reagent ion source at 0°close to central axis A, i.e. force them on a flight path close to theone indicated by arrow 306 in FIG. 4.

The reaction chamber 413 in this exemplary embodiment comprises a seriesof ring electrodes with constant orifice diameters 414 and an adjacentseries of ring electrodes with decreasing orifice diameters 415. DCvoltages are applied across electrodes 414 and 415 so they act as anIMR/PTR-MS drift tube. Additionally applied RF voltages allow forfocusing the ions and thus prevent ion losses. The reaction chamber isevacuated to between 1 and 10 hPa by a vacuum pump connected to apumping port in 413. Eventually, the ions are transferred into region416, which represents a differential pumping region, a TOF mass analyzerand a microchannel plate detector.

With the present invention sample gas can be introduced in-line with thecentral axis of the reaction chamber of an IMR/PTR-MS instrument.Although the inlet gas flow as well as the pressure in the reactionchamber can be fully controlled, the sample gas does not pass any valve,mass flow controller or similar device on its way into the reactionchamber. Moreover, there are no bendings or kinks the sample gas has topass, but it can directly enter the reaction chamber. This greatlyimproves the instrument's response and decay time because wall-effects,such as sample-wall interactions, are suppressed. In combination withthe possibility of extremely rapid reagent ion switching, because of atleast two reagent ion sources simultaneously producing reagent ions, anIMR/PTR-MS instrument according to the present invention is much faster,more selective and sensitive compared to existing designs.

By switching off all reagent ion sources or setting them so that none ofthem injects reagent ions into the reaction region, the front endaccording to the present innovation acts as an APi. This is onlypossible because the sample gas inlet design suppresses contact betweenthe sample gas and walls, which would inevitably lead to the loss of(atmospheric) ions. Thus, an instrument equipped with this front end isextremely cost efficient as two types of instrumentation are combined inone: APi-MS and IMR/PTR-MS.

The invention claimed is:
 1. An apparatus for Ion-Molecule-Reaction MassSpectrometry and/or Proton-Transfer-Reaction Mass Spectrometry,comprising: a sample gas inlet; an ion source section; a first reagention source; a second reagent ion source; a reaction chamber; and a massanalyzer, wherein the reaction chamber and the mass analyzer arearranged along a central axis, wherein the sample gas inlet is arrangedto introduce gas essentially along the central axis into the ion sourcesection that is connected to the reaction chamber, wherein the reactionchamber is placed downstream and essentially adjacent to the ion sourcesection, wherein the first reagent ion source and the second reagent ionsource are arranged essentially in a plane in front of the reactionchamber so as to emit reagent ions essentially perpendicularly to thecentral axis into the ion source section, said apparatus furthercomprising at least one electrode, such that the reagent ions emittedfrom the first or second reagent ion source into the ion source sectioncan be deflected into the reaction chamber essentially in the downstreamdirection of the central axis, wherein each reagent ion source comprisesat least one electrode with which the injection of reagent ions into theion source section and/or rejection back into the respective reagent ionsource is feasible, wherein said electrodes are connected to a switchingdevice so that emitted reagent ions from one reagent ion source aredeflected into the central axis while reagent ions from any otherreagent ion source are prevented from entering the reaction chamber. 2.The apparatus according to claim 1, wherein at least one further reagention source is arranged so as to emit reagent ions essentiallyperpendicularly to the central axis.
 3. The apparatus according to claim1, wherein the reagent ion sources are arranged essentially in a planewhich is essentially perpendicular to the central axis of the reactionchamber.
 4. The apparatus according to claim 2, wherein each one of thethree reagent ion sources is capable of producing one type of ions outof the group of H3O+, O2+ and NO+, wherein each one of the three reagention sources is configured to produce a different type of ions.
 5. Theapparatus according to claim 1, wherein the apparatus comprises a sampleinlet bypass line, wherein the sample inlet bypass line is arrangedessentially perpendicular to the sample gas inlet.
 6. The apparatusaccording to claim 5, wherein the apparatus comprises a gas line,wherein the gas line is arranged essentially perpendicular to the samplegas inlet.
 7. The apparatus according to claim 1, wherein at least oneelectrode is positioned opposite to each reagent ion source and/or atleast one electrode is positioned upstream in the vicinity of the samplegas inlet and one electrode is positioned downstream in the vicinity ofthe reaction chamber.
 8. The apparatus according to claim 7, whereineach reagent ion source comprises at least one electrode at its exit. 9.A method to operate an apparatus for Ion-Molecule-Reaction MassSpectrometry and/or Proton-Transfer-Reaction Mass Spectrometry accordingto claim 8, comprising the following steps: introducing sample gas intothe ion source section via the sample gas inlet in the direction of thecentral axis of the reaction chamber; continuously generating at leasttwo different reagent ions in the at least two reagent ion sources,wherein in one respective reagent ion source one specific type ofreagent ions is produced; applying a voltage to the at least oneelectrode at the exit of each reagent ion source, wherein a certainvalue of voltage leads to injection of the respective reagent ions intothe ion source section and another certain value of the voltage leads torejection back into the respective reagent ion source; applying avoltage to the at least one electrode positioned upstream in thevicinity of the sample gas inlet, wherein this electrode causes arepulsive force onto the reagent ions, and applying a voltage to the atleast one electrode positioned downstream in the vicinity of thereaction chamber, wherein this electrode causes an attracting force ontothe reagent ions, such that the reagent ions are injected into thereaction chamber essentially in the downstream direction of the centralaxis (A) of the reaction chamber; introducing the reagent ions and thesample gas into the reaction chamber, wherein subsequent to the reactionchamber the ions are analyzed with the mass analyzer.
 10. The methodaccording to claim 9, the method comprising applying a voltage to the atleast one electrode opposite to the at least two ion sources, saidelectrode causing a repulsive force onto the reagent ions.
 11. Themethod according to claim 9, characterized in that a controlling devicecontrols the voltages applied to the electrodes, wherein the controllingdevice controls which kind of the reagent ions generated in therespective ion source will be injected into the ion source sectionand/or which kind of reagent ions generated in the respective reagention source will be rejected back into the respective ion source.
 12. Theapparatus according to claim 1, wherein the apparatus comprises acontrolling device.
 13. A method to operate an apparatus forIon-Molecule-Reaction Mass Spectrometry and/or Proton-Transfer-ReactionMass Spectrometry according to claim 10 comprising the following steps:introducing sample gas into the ion source section via the sample gasinlet in the direction of the central axis of the reaction chamber;continuously generating at least two different reagent ions in the atleast two ion sources, wherein in one respective ion source one specifictype of reagent ions is produced; applying a voltage to the at least oneelectrode at the exit of each ion source, wherein a certain value ofvoltage leads to injection of the respective reagent ions into the ionsource section and another certain value of the voltage leads torejection back into the respective ion source; applying a voltage to theat least one electrode positioned upstream in the vicinity of the samplegas inlet, wherein this electrode causes a repulsive force onto thereagent ions, and applying a voltage to the at least one electrodepositioned downstream in the vicinity of the reaction chamber, whereinthis electrode causes an attracting force onto the reagent ions, suchthat the reagent ions are injected into the reaction chamber essentiallyin the downstream direction of the central axis of the reaction chamber;introducing the reagent ions and the sample gas into the reactionchamber, wherein subsequent to the reaction chamber the ions areanalyzed with the mass analyzer, wherein a controlling device controlsthe voltages applied to the electrodes, wherein the controlling devicecontrols which kind of the reagent ions generated in the respective ionsource will be injected into the ion source section and/or which kind ofreagent ions generated in the respective ion source will be rejectedback into the respective ion source, wherein the controlling deviceadapts the parameters of the apparatus, such that the apparatus ondemand acts as an Atmospheric Pressure interface Mass Spectrometer,wherein the parameters comprise at least the voltages applied to theelectrodes, wherein the controlling device controls that the ionsgenerated in the ion sources are rejected back into the respective ionsources or the ion sources are switched off by the controlling devicesuch that no reagent ions are generated.